I24 serial: Tidy up detector metadata devices

[noemifrisina]

Instead on instatiating first a beam_center device for both eiger and pilatus and then an additional pilatus_metadata device, there should be just one metadata device for the pilatus which inherits from the beamcenter and is instatiated at the start of the main plan.

• Rename beam_center_device to metadata_device
• Eiger only still has beam center logic
• Pilatus also has the filename template stuff